From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gdb-return-23158-listarch-gdb=sources.redhat.com@sourceware.org>
Received: (qmail 4805 invoked by alias); 11 Nov 2005 13:22:08 -0000
Mailing-List: contact gdb-help@sourceware.org; run by ezmlm
Precedence: bulk
List-Subscribe: <mailto:gdb-subscribe@sourceware.org>
List-Archive: <http://sourceware.org/ml/gdb/>
List-Post: <mailto:gdb@sourceware.org>
List-Help: <mailto:gdb-help@sourceware.org>, <http://sourceware.org/ml/#faqs>
Sender: gdb-owner@sourceware.org
Received: (qmail 4794 invoked by uid 22791); 11 Nov 2005 13:22:06 -0000
Received: from zigzag.lvk.cs.msu.su (HELO zigzag.lvk.cs.msu.su) (158.250.17.23)
    by sourceware.org (qpsmtpd/0.30-dev) with ESMTP; Fri, 11 Nov 2005 13:22:06 +0000
Received: from Debian-exim by zigzag.lvk.cs.msu.su with spam-scanned (Exim 4.50)
	id 1EaYqv-0003cZ-ML
	for gdb@sources.redhat.com; Fri, 11 Nov 2005 16:22:03 +0300
Received: from zigzag.lvk.cs.msu.su ([158.250.17.23])
	by zigzag.lvk.cs.msu.su with esmtp (Exim 4.50)
	id 1EaYqv-0003cO-If
	for gdb@sources.redhat.com; Fri, 11 Nov 2005 16:22:01 +0300
From: Vladimir Prus <ghost@cs.msu.su>
To: gdb@sources.redhat.com
Subject: read watchpoints ignored?
Date: Fri, 11 Nov 2005 13:22:00 -0000
User-Agent: KMail/1.7.2
MIME-Version: 1.0
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200511111622.01337.ghost@cs.msu.su>
X-SW-Source: 2005-11/txt/msg00228.txt.bz2


Hi,
I found a situation when read watchpoints are ignored by gdb. Consider this
program:

   #include <stdio.h>

   int a, b, c;

   int main()
   {
       a = 10;
       printf("Here I am\n");

       b = a;
       c = a;

       return 0;
   }

and this gdb session:

    (gdb) b main
    Breakpoint 1 at 0x80483a4: file rw.cpp, line 8.
    (gdb) r
    Starting program: /tmp/a.out

    Breakpoint 1, main () at rw.cpp:8
    8           a = 10;
    (gdb) rwatch a
    Hardware read watchpoint 2: a
    (gdb) c
    Continuing.
    Hardware read watchpoint 2: a

    Value = 10
    0x080483bd in main () at rw.cpp:11
    11          c = a;

Expected result: gdb stops on "b = a" line.
Actual result: gdb stops on "c = a".

Here's why it happens. When breakpoint is set, gdb reads the current value
of 'a', which is zero. After continue, a is assigned the value of '10', and
then first read watchpoint fires (on "b = a"). We arrive to
"bpstat_stop_status" (breakpoint.c), which has the following code:


  else if (b->type == bp_read_watchpoint || 
             b->type == bp_access_watchpoint)
      {
        ...
        if (found)
          {
            ...
            int e = catch_errors (watchpoint_check, bs, message,
                                  RETURN_MASK_ALL);
            switch (e)
              {
              .......
              case WP_VALUE_CHANGED:
                if (b->type == bp_read_watchpoint)
                  {
                    /* Don't stop: read watchpoints shouldn't fire if
                       the value has changed.  This is for targets
                       which cannot set read-only watchpoints.  */
                    bs->print_it = print_it_noop;
                    bs->stop = 0;
                    continue;
                  }


Since value of 'a' was changed by the "a = 10" line, "watchpoint_check"
returns "WP_VALUE_CHANGED", and reporting of watchpoint is completely
suppressed.

Questions:
1. Is this a bug?
2. Can this bug be fixed by removing this inner

      if (b->type == bp_read_wathcpoint)

   statement? This will cause watchpoints to trigger more often
   on those target that don't have "read-only watchpoints", but 
   there will be no risk of missing watchpoint. And I think missed
   watchpoint is more harm then spurious watchpoint.

Opinions?

- Volodya